An electrode is the component in an electrochemical cell in contact with the electrolyte through which current can flow by electronic movement. Electrodes, which are essential components of both galvanic (current producing) and electrolytic (current using) cells, can be composed of a number of electrically conductive materials, e.g., lead, zinc, aluminum, copper, iron, nickel, mercury, graphite, gold, or platinum. Examples of electrodes are found in electric cells, where they are dipped in the electrolyte; in medical devices, where the electrode is used to detect electrical impulses emitted by the heart or the brain; and in semiconductor devices, where they perform one or more of the functions of emitting, collecting, or controlling the movements of electrons and ions.
The electrolyte can be any substance that provides ionic conductivity, and through which electrochemically active species can diffuse. Electrolytes can be solid, liquid, or semi-solid (e.g., in the form of a gel). Common electrolytes include sulfuric acid and sodium chloride, which ionize in solution. Electrolytes used in the medical field must have a pH which is sufficiently close to that of the tissue in contact with the electrode (e.g., skin) so as not to cause harm to the tissue over time.
Electrochemically active species that are present in the electrolyte can undergo electrochemical reactions (oxidation or reduction) at the surface of the electrode. The rate at which the electrochemical reactions take place is related to the reactivity of the species, the electrode material, the electrical potential applied to the electrode, and the efficiency at which the electrochemically active species is transported to the electrode surface.
In unstirred electrolytes, such as quiescent liquid solutions and gel electrolytes, diffusion is the main process of transport of electrochemically active species to the electrode surface. The exact nature of the diffusion process is determined by the geometry of the electrode (e.g., planar disk, cylindrical, or spherical), and the geometry of the electrolyte (e.g., semi-infinite large volume, thin disk of gel, etc.). For example, diffusion of electrochemically active species to a spherical electrode in a semi-infinite volume of electrolyte differs from diffusion of electrochemically active species to a planar disk electrode. A constant and predictable pattern of diffusion (i.e., a diffusion pattern that can be predicted by a simple equation) is critical in determining a correlation between the electrochemical current collected, and the concentration of the electrochemically active species in the electrolyte.
However, diffusion of electrochemically active species toward an electrode can not be predicted by a simple equation for every situation. For example, where the electrochemically active species diffuses through a disk-shaped electrolyte toward a smaller disk-shaped electrode in contact with the electrolyte, the current observed at the electrode can not be predicted by a simple equation. In this latter situation, the inaccuracy in the diffusion model is caused by the combination of two different diffusion models. First, in the center of the disk electrode the diffusion of the electroactive species towards the electrode is in a substantially perpendicular direction. Secondly, at the edges of the disk electrode the diffusion comes from both perpendicular and radial directions. The combination of these two different diffusion patterns makes the total current collected at the disk electrode difficult to predict. In addition, the relative contributions of the diffusion fluxes from the axial and radial directions may change over time causing further errors in predicted current.